1. Field of the Invention
The dehydrogenation of hydrocarbons is an important commercial process. This is because of the great demand for dehydrogenated hydrocarbons as feedstocks for industrial processes. For example, dehydrogenated hydrocarbons are utilized in the manufacture of various products such as detergents, high octane gasolines, and pharmaceutical products among others. Plastics and synthetic rubbers are other products which may be produced through use of dehydrogenated hydrocarbons. One example of a specific dehydrogenation process is dehydrogenating isobutane to produce isobutene which may be etherified to produce gasoline octane improvers, polymerized to provide adhesive tackifying agents, viscosity-index additives and plastic anti-oxidants.
2. Related Art
Various reticulated ceramic structures are described in the art: U.S. Pat. No. 4,251,239 discloses fluted filter of porous ceramic having increased surface area; U.S. Pat. No. 4,568,595 discloses reticulated ceramic foams with a surface having a ceramic sintered coating closing off the cells; U.S. Pat No. 3,900,646 discloses ceramic foam with a nickel coating followed by platinum deposited in a vapor process; U.S. Pat. No. 3,957,685 discloses nickel or palladium coated on a negative image ceramic metal/ceramic or metal foam; U.S. Pat. No. 3,998,758 discloses ceramic foam with nickel, cobalt or copper deposited in two layers with the second layer reinforced with aluminum, magnesium or zinc; U.S. Pat. Nos. 4,810,685 and 4,863,712 disclose negative image reticulated foam coated with active material, such as, cobalt, nickel or molybdenum coating; U.S. Pat. No. 4,308,233 discloses a reticulated ceramic foam having an activated alumina coating and a noble metal coating useful as an exhaust gas catalyst; U.S. Pat. No. 4,253,302 discloses a foamed ceramic containing platinum/rhodium catalyst for exhaust gas catalyst; and U.S. Pat. No. 4,088,607 discloses a ceramic foam having an active aluminum oxide layer coated by a noble metal containing composition such as zinc oxide, platinum and palladium.
The supports employed in the present invention are generally of the type disclosed in U.S. Pat. No. 4,810,685 using the appropriate material for the matrix and are generally referred to in the art and herein as xe2x80x9cmonolithsxe2x80x9d.
The monoliths with various catalytic materials deposited thereon have also been employed for the production of synthesis gas (PCT WO 90/06279) and nitric acid (U.S. Pat. No. 5,217,939).
U.S. Pat. No. 4,940,826 (Freide, et al) discloses the oxidative dehydrogenation of gaseous paraffinic hydrocarbons having at least 2 carbon atoms or a mixture thereof by contacting the hydrocarbon with molecular oxygen containing gas over a supported platinum catalyst where the support is alumina such as gamma alumina spheres and monoliths such as cordierite or mullite. The desired products are the corresponding olefins.
Briefly the present invention is a process for the production of a mono-olefin from a gaseous paraffinic hydrocarbon having at least two carbon atoms or mixtures thereof comprising reacting said hydrocarbons and molecular oxygen in the presence of a platinum catalyst, preferably in the substantial absence of Pd, Rh and Au on a monolith support.
The composition of the ceramic support can be any oxide or combination of oxides that is stable at the high temperatures of operation, near 1000xc2x0 C. The support material should have a low thermal expansion coefficient. The components of the oxide support should not phase separate at high temperatures since this may lead to loss of integrity. Components of the oxide support should not become volatile at the high reaction temperatures. Suitable oxide supports include the oxides of Al (xcex1-Al2O3), Zr, Ca, Mg, Hf, and Ti. Combinations of these can be produced to tailor the heat expansion coefficient to match the expansion coefficient of the reactor housing.
The structure and composition of the support material is of great importance. The support structure affects the flow patterns through the catalyst which in turn affects the transport to and from the catalyst surface and thus the effectiveness of the catalyst. The support structure should be macroporous with 30 to 80 pores per linear inch. The pores should yield a tortuous path for the reactants and products such as is found in foam ceramics. Straight channel extruded ceramic or metal monoliths yield suitable flow dynamics only if the pore size is very small with  greater than 80 pores per linear inch.
The preferred catalyst of the present invention consists essentially of platinum supported on a ceramic foam monolith, preferably on zirconia or xcex1-alumina, and more preferably on zirconia. The platinum should be deposited on the surface of the ceramic to a loading of 0.2 to 90 wt. %, preferably 2 to 10 wt. %, and more preferably in the absence of palladium, rhodium, and gold. It has been found that palladium causes the catalyst to coke up and deactivate very quickly and thus should be excluded in any amount that is detrimental to the effectiveness of the catalyst. Though rhodium does not lead to catalyst deactivation the product distribution is less favorable. The presence of gold leads to a less active catalyst.
Preferably the Pt is supported on an alpha-alumina or zirconia ceramic foam monolith with 30 to 80 pores per linear inch, 30 to 70% void fraction, created in such a way to yield a tortuous path for reactants. The Pt may be supported on a ceramic foam monolith comprised of any combination of alpha-alumina, zirconia, titania, magnesia, calcium oxide, or halfmium oxide such that the support is stable up to 1100xc2x0 C. and does not undergo detrimental phase separation that leads to loss in catalyst integrity.
The catalyst may comprise platinum on the alumina or zirconia monolith support.